1. Field of the Invention
The present invention relates to an image forming lens, an image forming lens unit, an original document reading module, an original document reading method, an original document reading apparatus and an image information processing apparatus.
2. Description of the Related Art
An image forming lens is used in original document reading parts of various image forming apparatuses such as copying machines, facsimile machines, multi-functional products, etc. The image forming lens is also used in image scanners for reading an original document. The resolution of reading an original document with such apparatuses has recently increased from 300 or 400 dpi (dots per inch) to 600 dpi, and more recently to 1200 dpi. With such increase in the resolution of reading an original document with such apparatuses, higher performance is demanded for the lenses for use in reading the original document.
It is known that Gauss type lenses can accomplish a high resolution with a relatively wide lens diameter. The Gauss type lenses are widely used in original document reading parts of digital copying machines, multi-functional products, etc. and high-end flatbed image scanners. However, for realizing the higher performance recently demanded for an image forming lens, at least 6 pieces of lenses are required, as described for example in Japanese Patent Laid-open Publication No. 2000-111794. Therefore, the cost of such an image forming lens is inevitably high.
Japanese Patent Laid-open Publication No. 7-56085 describes an image forming lens having a two-lens construction. However, the image forming lens having a two-lens construction has a limitation in realizing such a higher performance recently demanded for the image forming lens. Therefore, the image forming lens having a two-lens construction is not suitable for use in an original document reading part of digital copying machines and multi-functional products requiring a relatively high resolution, and is generally used in facsimile apparatuses or image scanners not requiring a high reading resolution.
Also, an image forming lens having an anamorphic lens surface is known to be used in original document reading parts of digital copying machines, etc., as described for example in Japanese Patent Laid-open Publication No. 2000-307800. However, in the image forming lens of JP No. 2000-307800, the lens having the anamorphic lens surface is not arranged close to the image surface of the image forming lens, and therefore the effect of the anamorphic lens surface is not sufficiently realized.
For enhancing the performance of an image forming lens, it is conceivable to form a surface thereof in a non-spherical shape. Further, when the above-described original document reading parts of digital copying machines, etc. and the image scanners include a one-dimensional light receiving element array (line sensor), in addition to forming a surface of the image forming lens in a non-spherical shape, another surface thereof can be formed in an anamorphic shape, as described in Japanese Patent Laid-open Publication No. 2000-307800. However, as described above, the effect of the anamorphic lens surface is not sufficiently realized.
The present invention has been made in view of the above-discussed and other problems and addresses the above-discussed and other problems.
Preferred embodiments of the present invention provide a novel image forming lens that has a lesser number of lenses in comparison with Gauss type lenses and that yet has a performance higher than or equivalent to that of the Gauss type lenses.
Other preferred embodiments of the present invention provide an image forming lens that has the function and performance exceeding those of image forming lenses having a lens surface of an anamorphic shape and that is yet relatively low in cost.
According to a preferred embodiment of the present invention, an image forming lens for forming an image of an original document includes a front side lens group arranged at an object side and a rear side lens group arranged at an image forming side. The front side lens group includes from 2 to 4 individual lenses including at least one positive lens, and the rear side lens group includes one negative lens. The back-focus of the image forming lens when in use is equal to or smaller than 25% of an entire length of the image forming lens, and the air space, i.e., open space, between the front side lens group and the rear side lens group is equal to or greater than 50% of the entire length of the image forming lens. The front side lens group lens can include two, three or four lenses. By configuring an image forming lens with three individual lenses, correction of aberration is easy and relatively high performance can be realized.
In the above-described image forming lens, each lens can be spherical, so that manufacturing of each lens is relatively easy and thereby the image forming lens is realized at relatively low cost.
The above-described image forming lens can be configured such that at least one lens surface thereof is non-spherical. The non-spherical surface can be the incident side surface of the rear side lens group or the imaging side surface of the rear side lens group. That is, the rear side lens group can be configured such that the object side surface and/or the imaging side surface of thereof are non-spherical.
In the above-described image forming lens, the rear side lens group can be of plastic construction. In the above-described image forming lens, because the rear side lens group is disposed close to the image surface of the image forming lens, the rear side lens group is larger than the front side lens group. Accordingly, by using a plastic lens for the rear side lens group, the cost of the image forming lens can be further reduced. By using a plastic lens for the rear side lens group, the object side surface and/or the imaging side surface thereof can be easily formed in non-spherical shapes.
In the above-described image forming lens, because the rear side lens group is disposed close to the image surface of the image forming lens and a light receiving element array is arranged at the image surface, the outer shape of the rear side lens group can have a rotational asymmetry with respect to the optical axis of the image forming lens. In this case, the outer shape of the rear side lens group may be in a strip-like form having a long dimension in a main scanning direction. Here, the main scanning direction corresponds to the direction in which light receiving elements of the light receiving element array are arranged.
When the rear side lens group is plastic, an engaging part for positioning, supporting or fixing the image forming lens can be formed at a part of the rear side lens group. The engaging part can be protruding or concave.
In the above-described image forming lens, the back-focus of the image forming lens in the actual usage state thereof can be equal to or smaller than 10% of the entire length of the image forming lens. The smaller back-focus is advantageous in correcting aberration. However, if the back-focus is relatively large within the limit of 25% of the entire length of the image forming lens, for example, an adjusting mechanism for adjusting the position of a light receiving element array relative to the rear side lens group can be advantageously arranged within the back-focus.
In the above-described image forming lens, as described above, the outer shape of the rear side lens group can have a rotational asymmetry with respect to the optical axis of the image forming lens. However, the above-described image forming lens can be configured such that the outer shapes of the front side lens group and the rear side lens group have a rotational symmetry with respect to the optical axis of the image forming lens and thereby the entire image of an original document can be read at one time with an area sensor.
In the above-described image forming lens, for realizing high performance, the balance between the focal length of the front side lens group fF and the focal length of the rear side lens group fR is important. Preferably, the ratio fR/fF should be in the range of xe2x88x921.8 less than fR/fF less than xe2x88x920.8.
According to another preferred embodiment of the present invention, an image forming lens unit includes the above-described image forming lens and a supporting device configured to support each lens of the front side lens group and the rear side lens group and to integrate the front side lens group and the rear side lens group with each other.
The supporting device can be configured such that a front side lens group supporting part for supporting the front side lens group and a rear side lens group mounting part for mounting the rear side lens group are integrally formed. Alternatively, the supporting device may include a front side lens group supporting device for supporting the front side lens group, a rear side lens group supporting device for supporting the rear side lens group and a connecting device to connect the front side lens group supporting device and the rear side lens group supporting device with each other. In each of the above-described cases, the front side lens group can be made so as to move in the optical axis direction of the image forming lens relative to the rear side lens group supporting device or to rotate around the optical axis of the image forming lens, or to move in the optical axis direction and rotate around the optical axis. When the front side lens group can move in the optical axis direction, the focusing can be made by movement of the front side lens group in the optical axis direction. When the front side lens group is rotatable around the optical axis, inclination of the image surface due to eccentricity in the image forming lens can be corrected by rotating the front side lens group.
In the above-described image forming lens unit, one or more folding mirrors for bending an optical path of the image forming lens can be provided between the front side lens group and the rear side lens group. Thereby, freedom in the layout of the image forming lens can he increased.
Further, the above-described image forming lens unit may include a shading device for correcting the brightness of an image at the image surface in the main scanning direction, between the front side lens group and the rear side lens group. In this case, the shading device can be a shielding plate having an opening of a predetermined shape. The folding mirror may have a reflecting surface of a predetermined shape so as to function as a shielding device to correct the brightness of an image at the image surface of the image forming lens in the main scanning direction. When the above-described image forming lens unit is configured such that the entire image of an original document can be read at one time with an area sensor, the shading device can be configured so as to correct the brightness of an image on the area sensor.
According to another preferred embodiment of the present invention, an original document reading module for forming an image of an original document and converting image information of the image into electrical signals includes an image forming lens and a light receiving element array. The image forming lens is configured as described above. The light receiving element array is configured to photo-electrically convert image information of the image of the original document formed by the image forming lens on the light receiving element array into the electrical signals.
In the above-described image forming lens module, the rear side lens group and the light receiving element array can be integrated with each other. Further, when the back-focus of the image forming lens in the actual usage state thereof is relatively small, the image forming lens module may be configured such that the imaging side surface of the rear side lens group functions as a shielding member of a package of the light receiving element array. In particular, when the back-focus of the image forming lens is substantially zero, that is, the imaging side surface of the rear side lens group coincides with the image surface of the image forming lens, the light receiving element array may be directly attached to the imaging side surface of the rear side lens group. In this case, the imaging side surface of the rear side lens group can be non-spherical.
In the above-described image forming lens module, the supporting device can be configured in substantially the same manner as described above with respect to an image forming lens unit according to a preferred embodiment of the present invention.
Further, in the above-described image forming lens module, one or more folding mirrors for bending an optical path of the image forming lens can be provided between the front side lens group and the rear side lens group. Furthermore, the above-described image forming lens module may include a shading device for correcting the brightness of an image at the image surface in the main scanning direction, between the front side lens group and the rear side lens group. In this case, the shading device can be a shielding plate having an opening of a predetermined shape. The folding mirror may have a reflecting surface of a predetermined shape so as to function as a shielding device to correct the brightness of an image at the image surface of the image forming lens in the main scanning direction. When the above-described image forming lens unit is configured such that the entire image of an original document can be read at one time with an area sensor, the shading device can be configured so as to correct the brightness of an image on the area sensor.
According to another preferred embodiment of the present invention, an original document reading apparatus includes a scanning device configured to scan an original document; a light receiving element array configured to photo-electrically convert image information of an image formed thereupon into electrical signals; an image forming lens configured to image a light flux from the original document being scanned by the scanning device so as to form an image of the original document on the light receiving element array; a signal processing device configured to signal-process the electrical signals of the light receiving element array; and an image processing device configured to apply image processing to the electrical signals processed by the signal processing device. The image forming lens is configured as described above.
In the above-described original document reading apparatus, the image forming lens may include a supporting device configured to support each lens of the front side lens group and the rear side lens group and to integrate the front side lens group and the rear side lens group with each other, and the image forming lens and the supporting device may be integrated with each other so as to form the image forming lens unit described above.
Further, in the above-described original document reading apparatus, the image forming lens and the light receiving element array may be integrated with each other so as to form the original document reading module described above.
Furthermore, in the above described original document reading apparatus, the scanning device may include an original document contact glass provided at a predetermined position, a conveying device configured to convey the original document contacting the original document contact glass over the contact glass, and an illuminating device configured to illuminate the original document being conveyed over the contact glass across a main scanning direction of the original document. Alternatively, the scanning device may include an original document glass on which the original document is flatly placed at a predetermined position, an illuminating device configured to illuminate the original document placed at the predetermined position on the contact glass across a main scanning direction of the original document, a series of mirrors for guiding a light flux from a part of the original document illuminated by the illuminating device to the image forming lens through an optical path from the illuminated part of the original document to the image forming lens, and a moving device configured to move the illuminating device in a sub-scanning direction and to move the series of mirrors so as to maintain the optical path from the illuminated part of the original document to the image forming lens at a constant length.
In the above-described original document reading apparatus, the signal processing device or the image processing device may be configured to process the electrical signals of the light receiving element array such that a size of the image of the original document formed by the image forming lens on the light receiving element array is electrically adjusted. Alternatively, the signal processing device and the image processing device may be configured to process the electrical signals of the light receiving element array such that a size of the image of the original document formed by the image forming lens on the light receiving element array is electrically adjusted.
According to still another preferred embodiment of the present invention, an image information processing apparatus includes an original document reading device configured to read an image of an original document to convert image information of the image into electrical signals and an outputting device configured to output the electrical signals of the original document reading device. The original document reading device is configured as described above.
In the above described image information processing apparatus, the outputting device may include a display device to display the image information, such as a CRT, a liquid crystal display device or a liquid crystal panel of a liquid crystal projector, or a writing device to write the image information onto an electrical recording medium, such as a magnetic tape, a magnetic disc or an optical disc. Further, the outputting device may include a transmitting device to transmit the image information over a wired or wireless communication network. The outputting device may include a printer configured to print the image information on a recording medium formed in a sheet, such as paper or an overhead projector (OHP) sheet. The printer can be an ink jet printer or an electrophotographic optical printer. Further, the outputting device may include two or more of the above-described outputting devices. For example, the outputting device may include the above-described transmitting device, the writing device and the printer. In this case, the image information processing device can be a multi-functional apparatus having the function of reading the image of an original document and writing image information of the image into a magnetic or optical disc, the function of transmitting the image information via facsimile transmission, and the function of forming the image information on a sheet-like recording medium.
According to still another preferred embodiment of the present invention, an image forming lens for forming an image of an original document includes a front side lens group having 2 or more lenses and arranged at an object side, said 2 more lenses including at least one positive lens; and a rear side lens group including one negative lens and arranged at an image forming side. The back-focus of the image forming lens in an actual usage state thereof is equal to or smaller than 25% of an entire length of the image forming lens, an open space between the front side lens group and the rear side lens group is equal to or greater than 50% of the entire length of the image forming lens, and the rear side lens group includes at least one anamorphic surface.
According to still another preferred embodiment of the present invention, a method of reading an image of an original document includes the steps of providing an image forming lens configured as described above; scanning the original document and forming the image of the original document on a light receiving element array with the image forming lens; and converting image information of the image of the original document formed on the light receiving element array by the image forming lens into electrical signals with the light receiving element array.
In the above-described method, the step of providing the image forming lens may include the step of providing a supporting device for supporting each lens of the front side lens group and the rear side lens group and for integrating the front side lens group and the rear side lens group with each other so as to form an image forming lens unit. In this case, in the scanning and image forming step, the original document is scanned and the image of the original document is formed on the light receiving element array with the image forming lens unit.
Further, the above-described method may further include the step of integrating the rear side lens group of the image forming lens and the light receiving element array with each other so that the image forming lens and the light receiving element array forms an original document reading module. In this case, in the scanning and image forming step, the original document is scanned and the image of the original document is formed on the light receiving element array of the original document reading module, and in the converting step, the image information of the image of the original document is converted into the electrical signals with the light receiving element array of the original document reading module.
Furthermore, in the above-described method, the scanning and image forming step may include the step of illuminating the original document at an original document illuminating part at a predetermined position with an illuminating device while conveying the original document relative to the original document illuminating part. Alternatively, in the above-described method, the scanning and image forming step may include the step of illuminating the original document in the main scanning direction with an illuminating device while maintaining a fixed optical path length from the image forming lens to the light receiving element array, guiding a light flux from a part of the original document illuminated by the illuminating device to the image forming lens via a series of mirrors, moving the illuminating device in the sub-scanning direction and moving the series of mirrors such that the optical path length from the illuminating device to the image forming lens is constant.